Porphyria cutanea tarda (PCT) is the most common of the porphyrias and is due to a marked deficiency of uroporphyrinogen decarboxylase in liver. The uroporphyrinogen decarboxylase deficiency found in PCT is primarily acquired. The aims of this study are as follows. 1) Vitamin C status will be assessed before and after treatment by measuring vitamin C levels in serum and white blood cells. This will test the hypothesis that vitamin C is an antioxidant in the liver and that its deficiency may contribute to activation of PCT. 2) Activity of CYP 1A2 will be assessed in vivo using 13C-caffeine and 13C-methacetin breath tests and also by measuring the clearance rate of caffeine from blood and urinary excretion of caffeine metabolites. 3) The role of hepatitis C infection will be assessed by (a) observing the effect of treatment of PCT by repeated phlebotomy or low-dose chloroquine on abnormalities in liver function tests and on the presence of hepatitis C antibody and hepatitis C RNA in plasma; and (b) by observing the frequency of recurrence of PCT in patients who are treated with interferon alpha-2b as compared to patients who are not treated with this drug. Standard serum chemistries as will as a 13C-phenylalanine breath test will be used to assess liver function. The indications for treatment with interferon alpha-2b will be the standard indications for use of this drug in patients with hepatitis C and will not be influenced or specified by this protocol. In all cases patients will be treated first for PCT, because treatment for PCT is almost always successful, whereas treatment for hepatitis C is usually not.